More melted fuel has been found at the bottom of the Fukushima power planet, seven years after Japan's worst nuclear disaster.

Tokyo Electric Power Company (TEPCO), the operator of Japan's crippled nuclear plant, says a long telescopic probe has successfully captured images of the fuel inside the plant's Unit 2 primary containment vessel.

The images showed that at least part of the fuel breached the core, falling to the vessel's floor.

TEPCO says that that the status inside the primary containment vessel is still stable, and that there are no changes in radiation levels at the site boundaries of Fukushima Daiichi's Nuclear Power Plant.

A massive earthquake and tsunami in 2011 caused three reactors at the Fukushima plant to melt.

Determining the location of the melted fuel is crucial in planning for its removal, the hardest process in the plant's decades-long decommissioning.

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A photo taken by a robotic probe on Friday, January 19, 2018, shows a view of the bottom of a structure housing a safety system called the control rod drive, which appeared rusty and coated with unidentified material at the Fukushima nuclear plant

Penetration at the pedestal area at the base of the reactor has been challenging due to deposits that have blocked the path of robots designed to crawl around it as well as high radiation levels that limit the life of electronic equipment.

These high radiation levels mean that the work must be done remotely to protect workers' safety, and precautions are also taken to protect the environment and make sure that no gases escape from the reactor during the tools' insertion.

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A photo taken by a robotic probe on Friday, January 19, shows a part of what is believed to be the handle of the fuel rods container and melted fuel in small lumps scattered on a structure below the Fukushima reactor core

Melted fuel has previously only been documented inside Unit 3, on July 21, 2017.

At the time, plant operator TEPCO said the robot found large amounts of lava-like debris apparently containing fuel that had flowed out of the core into the primary containment vessel of the Unit 3 reactor at Fukushima.

'This next effort at Unit 2 illustrates our commitment to developing technologies that will enable decommissioning at Fukushima Daiichi and also become useful elsewhere,' said Chief Decommissioning Officer Naohiro Masuda.

'Even with the new approach this will be a challenging mission, but we will persevere because obtaining this information is important to developing the ability to eventually remove the fuel debris.'

A photo taken by a robotic probe on Friday, January 19, shows part of a grated platform that broke and fell from a structure below the core of Unit 2 reactor's primary containment vessel coated with what could be melted fuel or structure inside the Fukushima nuclear plant

During the 2017 study, cameras mounted on the robot showed extensive damage caused by the core meltdown, with fuel debris mixed with broken reactor parts, suggesting the difficult challenges ahead in the decades-long decommissioning of the destroyed plant.

Experts have said the fuel melted and much of it fell to the chamber's bottom and is now covered by radioactive water as deep as 6 meters (20 feet).

The fuel, during meltdown, also likely melted its casing and other metal structures inside the reactor, forming rocks as it cooled.

The 2017 expedition relied on a marine robot the size of a loaf of bread dubbed the 'little sunfish.'

It was inserted along a rail and it included a camera, temperature sensors and a dosimeter to measure radiation directly.

FUKUSHIMA'S 'SUNFISH' ROBOT

The marine robot is on a mission to study damage and find resources inside the devastated plant.

The 2017 version of the marine robot, nicknamed the 'little sunfish', was about the size of a loaf of bread.

It was equipped with lights, manoeuvres using tail propellers and collects data using two cameras and a dosimeter radiation detector.

During a demonstration of the device at a test facility near Tokyo last year, the probe slowly slid down from a rail and moved across the water.

A team operated it remotely, with one guiding the robot while another adjusted a cable that transmits data and serves as its lifeline.

An underwater robot (pictured) captured images and other data inside Japan's crippled Fukushima nuclear plant in 2017. Pictured is the robot on its first day of work

While the investigation yielded valuable images and data, the images were limited by debris, including a fallen floor grate that obscured the view of the bottom of the pedestal.

In addition, the device's ability to penetrate further into the reactor was obstructed by deposits that had formed on the rail that guided the device into the reactor.

For its latest investigation, TEPCO improved its tools and used a different approach to try to see below the fallen grating into the bottom of the pedestal, where the researchers believe the formerly molten fuel debris sit.

The newly improved version of the bot includes:

A lengthened and strengthened guide pipe, and an extendable device added to help the camera and other electronic extend further into the pedestal area.

The lights and electronic measurement equipment at the tip of the pipe were made smaller.

A skirt has been added to the device to reduce the potential for snagging on objects, and a mechanism has been added to lower the device from the guide pipe down into the platform bottom.

An anti-fogging technology was applied to the camera to obtain clearer images.

The 'Little sunfish' (pictured), which is about the size of a loaf of bread, has now begun collecting in the highly radioactive waters

While the investigation yielded valuable images and data, the images were limited by debris, including a fallen floor grate that obscured the view of the bottom of the pedestal.

In addition, the device's ability to penetrate further into the reactor was obstructed by deposits that had formed on the rail that guided the device into the reactor.

So TEPCO improved its tools and used a different approach to try to see below the fallen grating into the bottom of the pedestal area, where the researchers believe the formerly molten fuel debris sit.

The improvements include a lengthened and strengthened guide pipe, and an extendable device added to help the camera and other electronic extend further into the pedestal area.

In addition, the lights and electronic measurement equipment at the tip of the pipe were made smaller.

A skirt has been added to the device to reduce the potential for snagging on objects, and a mechanism has been added to lower the device from the guide pipe down into the platform bottom.

Finally, an anti-fogging technology was applied to the camera to obtain clearer images.

An image lava-like lumps believed to contain melted fuel inside the Unit 3 reactor at Fukushima. This image was taken during July 2017 study

Speaking at the time of the 2017 study, TEPCO spokesman Takahiro Kimoto said it was the first time a robot camera has captured what is believed to be the melted fuel.

'We believe it is highly likely to be melted fuel or something mixed with it,' he said.

He said it would take time to analyze which portions of the rocks were fuel.

In an earlier survey, the robot found severe damage in the vessel, including key structures that were broken and knocked out of place..

Experts have said the melted fuel is most likely to have landed inside the pedestal after breaching the core.

Another image of lava-like lumps believed to contain melted fuel inside the Unit 3 reactor at Fukushima, taken in July 2017

Kimoto previously said that the robot probe has captured a great deal of useful information and images showing the damage inside the reactor, which will help experts eventually determine a way to remove the melted fuel, a process expected to begin sometime after the 2020 Tokyo Olympics.

'It's still just the beginning of the (decades-long) decommissioning,' said Kimoto.

'There is still a long way to go, including developing the necessary technology.'

'But it's a big step forward,' he said.

THE FUKUSHIMA DISASTER

In 2011, a 10-metre-high tsunami that killed nearly 19,000 people crashed into Japan's Fukushima nuclear power plant.

This lead to several meltdowns, allowing harmful radioactive fuel rods and debris to escape from contained areas.

Five years after the disaster, researchers are still struggling to clean up fuel in the waters of the wasting reactors.

An aerial view of the reactors of the tsunami-stricken Fukushima Dai-ichi nuclear power plant stand in Okuma, Fukushima

It's estimated that plant officials have only located 10 per cent of the waste fuel left behind after the nuclear meltdowns.

And the damaged plant is believed to be leaking small amounts of the radioactive waste into the Pacific Ocean, which could be travelling as far as the west cost of the US.

Researchers are now pinning their hopes on the remote-controlled sunfish robot to locate the lost fuel in order to work out the safest way to remove it.

Split down the middle: Workers inspect a section of road that was torn in half by the force of the earthquake